The present invention relates to an image forming apparatus, and more particularly relates to an electrostatic color copier by which a color image of a document can be formed on a recording paper.
In a conventional color image forming apparatus of the kind described above, the following principle is applied: toner is attracted to an electrostatic latent image which has been formed on the circumferential surface of a photoreceptor drum so that a toner image is formed; and the formed toner image is transferred onto a recording paper.
In this kind of image forming apparatus, the following transfer systems by which a toner image on a photoreceptor drum surface is transferred onto a recording paper, have been widely known.
(1) The conveyance transfer system which is characterized in that: an image is transferred onto a horizontally conveyed recording paper while the recording paper is squeezed by a photoreceptor drum and an idle roller. This system is mainly applied to monocolor copiers.
(2) The paper drum transfer system which is characterized in that: a paper drum is provided opposed to a photoreceptor drum; and a recording paper is tightly wound around the paper drum so that an image can be transferred onto the recording paper. This system is mainly applied to color copiers.
The above-described transfer systems have a disadvantage as follows.
(1)' In the case of the conveyance transfer system, an image can be formed on various sizes of paper. However, when the drum size is increased, there is a possibility that a recording paper can become stuck around the photoreceptor drum.
(2)' In the case of the paper drum system, it is suitable for forming a color image since multiple transfer can be conducted. However, this system can not be applied to various sizes of paper, or when thick paper is used as a recording paper.
As large and precise images are required recently, the transfer belt system has become a center of attention, wherein the transfer belt system is characterized in that a recording paper is electrostatically attracted onto the surface of a transfer belt. In the transfer belt system, there is no possibility that a recording paper becomes adhered onto the surface of a photoreceptor drum, so that the transfer belt system is effective when it is applied to a color copier which is provided with a large photoreceptor drum. Consequently, this system has been applied to many copiers.
In the case of a conventional image forming apparatus, the apparatus is composed in such a manner that: a transfer belt unit consisting of a transfer belt stretched around a paper entry side roller (an idle roller) and a delivery side roller (a drive roller) is provided; the transfer belt is pressed against the photoreceptor drum surface with pressure when transfer is conducted; and when transfer is not conducted, the transfer belt is separated from the photoreceptor drum by a cam mechanism provided on a support frame, wherein the transfer belt is rotated around the delivery roller. Then, the transfer belt comes into contact with the photoreceptor drum only when transfer is performed, and the wear of both the transfer belt and photoreceptor drum can be reduced.
Since the transfer belt unit is a heavy structure consisting of at least two rollers, one belt and a frame, a heavy load is given to the cam which moves the transfer unit up and down, so that it is difficult to move the transfer unit in quick response, and further there is a possibility that the cam becomes worn out and problems are caused in the cam drive mechanism.
Due to the above-described situation, it is a primary object of the present invention to provide an image forming apparatus in which the transfer belt can be contacted with and moved from the photoreceptor drum surface without giving too heavy a load to the cam mechanism.
In the case of the above-described conventional image forming apparatus, it is necessary to provide a toner recovery box to the transfer unit, which scrapes the residual toner adhered to the surface of the transfer belt and recovers the scraped toner.
This toner recovery box is fixed to the apparatus body through a delivery tube. On the other hand, the transfer belt unit needs to be attachably provided to the apparatus body so that it can be easily removed from the body when the transfer belt is exchanged during maintenance. Further, the transfer belt unit and the toner recovery box must form one body when copy operations are performed, so that toner can not be scattered from the gap between the transfer belt unit and the toner recovery box.
With a view to solving the problems described above, the second object of the present invention is to provide an image forming apparatus having a means to couple a transfer belt and a toner recovery box is provided, by which the toner recovery box can be easily separated from the transfer belt. Furthermore the means is simply composed and of low cost.